Heavy metal particles often occur as mixtures with sands and gravels. These particles must be separated from the mixture for recovery or safe disposal. For example, particles containing toxic heavy metals such as mercury are produced in medical, mining, and industrial operations, and must be removed from soils, sediments, and bodies of water to ensure the safety of the environment. Particles including gold or platinum occur naturally in soils and sediments, and are recovered for their commercial value.
Magnetic devices for the recovery of magnetically susceptible metal particles are well known. For example, U.S. Pat. No. 823,301 to Snyder discloses a magnetic separator including an inclined chute equipped with an array of magnets to separate particles traveling along the chute surface on the basis of their magnetic susceptibility. Such devices are of no use for the recovery of heavy metals that are not magnetically susceptible, including mercury, gold, and platinum.
Nonmagnetic heavy metal particles can be recovered with passive recovery systems, also known as gravity recovery systems. In a gravity recovery system, a mixture of particles is suspended as a slurry in a liquid medium, usually water, and the particles are allowed to sediment out according to their specific gravities. Many commonly used gravity recovery systems include a sluice box, a device which channels a flow of slurry over a series of riffles. A riffle is a baffle-like obstacle which resists the flow of slurry to create regions of reduced flow rate in the areas between the riffles. In these regions of reduced flow, the heaviest particles sediment out. Lighter particles continue in the flow over the top of the riffle. A bottom mat of natural or synthetic fiber or textured rubber or plastic is often situated upon the floor of a sluice box to trap the finer particulates after they have settled, and to prevent their being scoured back into suspension by larger passing particles or by a surge in the flow rate of the slurry.
Optimal recovery of metal particles from a sluice box recovery system requires that the height and shape of the riffles be adjusted according to the rate of slurry flow, the specific gravity of the metal particulate to be recovered, and the specific gravities of particulates to be rejected, that is, to be allowed to flow over the riffles and leave the sluice box. Existing sluice box systems include rigid linear riffles which provide no flexibility in riffle size, shape or distribution. There is a need for a gravity sedimentation system which provides riffles of variable geometric patterns and sizes.
The recovery of settled metal particulates from the riffles of a sluice box is also a cumbersome process which requires the disassembly of the sluice box, the washing out of the bottom mat, and the reassembly of the sluice box. There is a need for a gravity separation system wherein the riffles can be instantaneously disassembled and reassembled, without mechanical intervention.
Magnetite is a magnetically susceptible iron oxide that usually occurs in particulate form in the same sands and sediments as heavy metal particulates. Magnetic systems have been developed for the recovery of nonmagnetic heavy metal particles by virtue of their physical or chemical association with magnetite. For example, U.S. Pat. No. 6,596,182 to Prenger, et al. discloses a device for removing heavy metals from water, including a reaction chamber wherein heavy metals in the water are either adsorbed to magnetite particles or incorporated chemically into magnetite particles formed in situ. The water is then streamed through columns of magnetically charged steel mesh. The magnetite particles, and the heavy metals adsorbed or incorporated thereto, bind to the steel mesh for eventual recovery by flushing the column with water or air.
The use of magnetite in gravity separation devices has been disclosed in U.S. Pat. Nos. 5,927,508 and 7,811,088, both to Plath. In each of the disclosed devices, a flow of slurry is directed through a sluice box including riffles of rigid material. The invention of U.S. Pat. No. 7,811,088 also includes settling chambers to facilitate the sedimentation of heavy metal particles. A vinyl material impregnated with a weak magnetic compound in transverse rows is situated on the floor of the sluice box. Upon exposure to the magnetically assisted sluice, magnetite particles suspended in the slurry form a porous mat in contact with the magnetic material. This magnetite mat traps heavy metal particles after they have been induced to sediment by the rigid riffles or the settling chambers. Essentially, the magnetite mats of the devices disclosed by Plath serve the function of the fiber or textured plastic mat of conventional sluice boxes. Recovery of settled particles from the separation devices disclosed by Plath requires either the flushing out of the porous magnetite mat, against the force of the magnetic material and past the rigid riffles, or the disassembly of the separation device. The devices disclosed by Plath do not provide riffles of variable geometric patterns, sizes, and magnetic strengths, or riffles that can be instantaneously disassembled and reassembled without mechanical intervention.